1. Field of the Invention
The invention relates, generally, to a supporting cradle adapted for use with a lifting device and, more specifically, to a supporting cradle adapted to be used with a jack to lift and support the balance shaft assembly of a diesel engine.
2. Description of the Related Art
In certain types of diesel engines that are used for industrial purposes and for those used in motor vehicles, it is known to employ a counter shaft, or counter shaft assembly. These counter shaft assemblies, also referred to as balance shafts and balance shaft assemblies, are used to, provide counter balance, and thus smooth operation, to the crankshaft of the engine. Counter balance is required with certain engine configurations, such as four cylinder engines for example, where the combustion forces from the power stroke of each cylinder occur at high angular differences from each other.
In the case of the four-cylinder engine, the typical crankshaft configuration places the two end pistons and the two middle pistons in the same relative position within their relative cylinder bores at any given time. Thus, when the number 1 piston is at the top of its stroke at top dead center (TDC), then the number 4 piston is also at TDC. Concurrently, the number 2 and number 3 pistons will both be at bottom dead center (BDC). In four-stroke diesel engines, this lends itself well to the timing of the combustion for each cylinder, which produces the power stroke and imparts the motive force to the crankshaft. Specifically, as the number 1 piston reaches TDC in its compression stroke and the fuel/air mixture is ignited to begin the power stroke by driving the piston downward, the number 4 is piston is reaching TDC of its the exhaust stroke and will begin its intake stroke during power stroke of piston number 1. Subsequently, as the number 1 piston is driven downward in its power stroke, one of the rising pistons (typically piston number 3) is in its compression stroke and will start its power stroke as it reaches TDC and piston numbers 1 and 4 reach BDC.
In a four-stroke, four-cycle diesel engine, combustion and a new power stroke begins every 180 degrees of crankshaft rotation. Arranging the pistons in relation to each other in this manner allows the power stroke of each piston to be fully complete before starting the next combustion stroke. However, each power stroke generates additional vertical forces that are not passed to the crankshaft but are passed to the engine itself. Since the piston that is being driven downward in its power stroke by the combustion forces is moving faster than the piston rising in the compression stroke, the vertical forces in the engine block are not cancelled out. Additionally, the 180 degree differential in power strokes alternately creates angular force on the forward half of the crankshaft the first ½ rotation, then an angular force acting on the rear half of the crankshaft in the second ½ rotation. This fore and aft, combustion-generated, angular force each 180 degrees of crankshaft rotation causes end-to-end engine vibration. The vertical force vibration and the end-to-end vibration result in rough engine operation in the four-cylinder engine. This is especially true of large displacement four cylinder diesel engines when they are operated at low speeds. In contrast, six-cylinder engines have much more rotational inertia and can function well with the pistons angularly offset typically at 120 degree intervals, which allow the power strokes to occur 3 times for each revolution of the crankshaft providing inherently smooth operation, and negating the need for counter-balancing.
In order to overcome the inherent vibration and rough running problem in a four-cylinder, four-stroke diesel engine, it is known in the related art to employ a balance shaft assembly. One approach that has met with limited success involves the use of a single counterweighted balance shaft that is directly installed in the engine block. The balance shaft is operatively connected to and rotates with the crankshaft. However, a more successful approach to countering four-cylinder engine vibration has been to employ a separate multiple counter-balance shaft assembly that is operatively driven by the crankshaft. These separate balance shaft assemblies often include heavy castings that operatively retains two heavily weighted counter balance shafts driven in counter rotation by the crankshaft. The balance shaft assembly is physically located at the bottom of the engine block just above the oil pan and often incorporates the oil pump assembly into the same gearing that drives the balance shafts.
The weight of the balance shaft assembly successfully adds stabilizing inertia to the engine to counter the end-to-end vibration, as well as providing the rotational counterbalance to cancel the vertical vibration forces. This produces a smooth running four-cylinder diesel engine. However, whenever these engines must undergo maintenance, the heavy balance shaft assembly causes difficulties in its handling for removal and replacement. For example, in the case of a four-cylinder, four-cycle diesel engine, the balance shaft assembly is bolted to the bottom of the engine block and includes the oil pump and oil pump pickup tube which extends down into the oil pan. Any servicing of the oil pump or components located in the bottom end of the engine, such as main bearings or connecting rod bearings requires removal of the balance shaft assembly. If it is necessary to remove the engine from the vehicle, the balance shaft assembly is also generally removed first to reduce the engine weight and simplify engine removal.
Removal of the balance shaft assembly is problematic due to its weight and location as installed in a vehicle. Known methods of removal generally require the physical efforts of two or more individuals to handle the assembly and one other individual to loosen and remove the bolts holding it to the engine. Sometimes a jack may also be employed to assist in raising or lowering the balance assembly to or from the engine block. However, if the vehicle is sitting on its wheels or only raised up on jack stands for limited underneath access, the balance shaft assembly is physically close to the ground and it is difficult to get the necessary individuals or a jack under the vehicle to handle the assembly while it is being unbolted and removed or being reinstalled and bolted in. Additionally, a typical floor jack is inadequate to support the balance shaft assembly and can cause the assembly to drop to the floor.
Likewise, if the vehicle is raised up on a lift or hoist, the access to the balance shaft assembly is better but the weighty assembly is difficult to hold overhead and then lower to the ground as its is unbolted, or to lift and hold in it place when reinstalling it. Additionally, the oil pump pick up tube extends downward from the balance shaft assembly and it is easily damaged if the assembly is not carefully removed. If a jack is employed to assisting in holding the balance shaft assembly when the vehicle is on a lift or hoist, it is generally a transmission type jack due to its ability to reach the level of the engine. However, a transmission jack is not adequate to properly support the assembly. In this case, extreme care must be taken to avoid damaging the oil pickup tube or having the balance shaft assembly slide off of the jack and drop to the floor. At present, there are no known jack types or cradle assemblies that can be employed to properly assist in the removal and replacement of balance shaft assemblies.
Therefore, there exists a need in the art for a supporting fixture that can be used with existing jacking mechanisms to properly cradle and support a balance shaft assembly so that it can be easily raised and lowered to and from the engine block. There is also a need for such a fixture so that a single individual can raise and lower a balance shaft assembly to install or remove it.